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CVE-2026-81725— NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader

Quick assessment

Affected
nltk nltk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Natural Language Toolkit是Natural Language Toolkit组织开源的一个自然语言处理工具包。 Natural Language Toolkit 3.10.3之前版本存在资源管理错误漏洞，该漏洞源于Pl196xCorpusReader中的正则表达式拒绝服务，攻击者可通过向words()和tagged_words()等公共API提供带有大量未匹配开始标签的格式错误TEI块，利用read_block方法中的懒惰正则表达式模式强制重复扫描，导致CPU消耗呈二次方增长。

CVSS 3.7 · Low EPSS 0.37% · P29

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service

Affected Version Matrix 2

VendorProduct Version RangeStatus
nltk nltk < 3.10.3 affected
3.10.3 unaffected
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I. Basic Information for CVE-2026-81725

Vulnerability Information
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Vulnerability Title
NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader
Source: CVE Program / CVE List V5
Vulnerability Description
NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
未加控制的资源消耗(资源穷尽)
Source: CVE Program / CVE List V5
Vulnerability Title
Natural Language Toolkit 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Natural Language Toolkit是Natural Language Toolkit组织开源的一个自然语言处理工具包。 Natural Language Toolkit 3.10.3之前版本存在资源管理错误漏洞，该漏洞源于Pl196xCorpusReader中的正则表达式拒绝服务，攻击者可通过向words()和tagged_words()等公共API提供带有大量未匹配开始标签的格式错误TEI块，利用read_block方法中的懒惰正则表达式模式强制重复扫描，导致CPU消耗呈二次方增长。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
nltk nltk 0 ~ 3.10.3 -

II. Public POCs for CVE-2026-81725

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III. Intelligence Information for CVE-2026-81725

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Vendor Advisories for CVE-2026-81725 (2)

Same Patch Batch · nltk · 2026-08-27 · 6 CVEs total

CVE-2026-81722 7.5 HIGH nltk PorterStemmer before 3.10.3 Quadratic-time DoS
CVE-2026-81727 7.1 HIGH NLTK before 3.10.3 Hardlink File Overwrite via downloader
CVE-2026-81726 7.0 HIGH NLTK through 3.10.3 Path Traversal via Model-Artifact APIs
CVE-2026-81724 5.3 MEDIUM NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion
CVE-2026-81723 3.7 LOW NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusView

IV. Related Vulnerabilities

Same product: nltk
Same vendor: nltk
Same weakness: CWE-400

V. Comments for CVE-2026-81725

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